Ion implantation is a standard technique for introducing conductivity-altering impurities into semiconductor wafers. A desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the wafer. The energetic ions in the beam penetrate into the bulk of the semiconductor material and are embedded into the crystalline lattice of the semiconductor material to form a region of desired conductivity.
Ion implantation systems usually include an ion source for converting a gas or a solid material into a well-defined ion beam. The ion beam is mass analyzed to eliminate undesired ion species, is accelerated to a desired energy and is directed onto a target plane. The beam is distributed over the target area by beam scanning, by target movement or by a combination of beam scanning and target movement. An ion implanter which utilizes a combination of beam scanning and target movement is disclosed in U.S. Pat. No. 4,922,106 issued May 1, 1990 to Berrian et al.
The delivery of a parallel ion beam to the semiconductor wafer is an important requirement in many applications. A parallel ion beam is one which has parallel ion trajectories over the surface of the semiconductor wafer. In cases where the ion beam is scanned, the scanned beam is required to maintain parallelism over the wafer surface. The parallel ion beam prevents channeling of incident ions in the crystal structure of the semiconductor wafer or permits uniform channeling in cases where channeling is desired. Typically, a serial ion implanter is utilized when a high degree of beam parallelism is required.
In one approach, the beam is scanned in one dimension so that it appears to diverge from a point, referred to as the scan origin. The scanned beam then is passed through an ion optical element which performs focusing. The ion optical element converts the diverging ion trajectories to parallel ion trajectories for delivery to the semiconductor wafer. Focusing can be performed with an angle corrector magnet or with an electrostatic lens. The angle correction magnet produces both bending and focusing of the scanned ion beam. Parallelism may be achieved with an electrostatic lens, but energy contamination can be a drawback.
The output ion beam from the angle corrector magnet or other focusing element may be parallel or may be converging or diverging, depending on the parameters of the ion beam and the parameters of the focusing element. When an angle corrector magnet is utilized, parallelism can be adjusted by varying the magnetic field of the angle corrector magnet. The angle corrector magnet typically has a single magnetic field adjustment which varies both parallelism and bend angle, or beam direction. It will be understood that the ion implanter is often required to run a variety of different ion species and ion energies. When the beam parameters are changed, readjustment of the angle corrector magnet is required to restore beam parallelism.
In prior art ion implanters, the angle corrector magnet is typically adjusted so that the ion beam has normal incidence on a wafer plane of the ion implanter end station. However, the angle corrector adjustment which achieves normal incidence on the wafer plane may result in less than optimum parallelism. In particular, an ion beam that is adjusted for normal incidence on the wafer plane may be somewhat diverging or converging. As shown in FIG. 8, the angle corrector magnet is adjusted such that the center ray of ion beam 200 is normal to wafer plane 202. However, when the beam 200 is adjusted to be normal to wafer plane 202, the parallelism of beam 200 may be degraded such that the beam converges or diverges. The lack of parallelism is unacceptable in highly critical applications.
In another approach, the angle corrector magnet is designed for best parallelism under typical conditions, and the ion implanter end station is positioned for normal incidence of the ion beam on the wafer. However, beam parallelism and normal incidence are not maintained over a wide range of beam parameters, and changing the position of the end station is very difficult.
Accordingly, there is a need for improved methods and apparatus for adjusting beam parallelism in ion implanters.